A small, laden coaster with a pilot embarked unberthed and turned at rest, before proceeding downriver towards the open sea.
The departure had been timed to coincide with the last of the food tide and the passage plan allowed for a minimum under keel clearance of 1m.
Having completed the turn, the vessel started to make way along the channel. When approaching the first bend in the river, the pilot applied starboard wheel, but the vessel did not respond.
More wheel was soon applied and the vessel started to turn slowly to starboard.
However, by this time, the vessel was on the port side of the channel and its stern struck a small, moored leisure vessel during the turn.
There were three people onboard the leisure vessel at the time of the collision, but thankfully none got injured.
- Hydrodynamic effects can make vessels difficult to turn when the depth and width of a channel are restricted, and these infuences should always be taken into account when passage planning.
- In this case, once the vessel was of the centerline of the channel, it was likely to have been subject to a squatting effect, which can make steering sluggish.
- It is also likely that a ‘bank effect’ was experienced, creating a suction zone around the stern, causing it to swing to port towards the moored leisure vessels.
- Methods to reduce the effects of operating in shallow and narrow channels include reducing speed, increasing wheel, planning to apply rudder early, and maximising under keel clearance in the passage plan.
- After this incident, the channel was surveyed to make sure that the best possible local information was available for planning.
- This accident also illustrates the inherent risk to leisure craft moored close to navigational channels used by commercial vessels.
In this case the harbourmaster had sensibly placed a condition on leisure berth holders, prohibiting overnight stays on board. In addition, after the collision, arrangements were put in place for the port’s workboat to escort commercial vessels and warn leisure users on the river of the approaching vessel.
…as the Safety Digest concluded.